1. Technical Field
This disclosure relates to Electron Beam Physical Vapor Deposited Thermal Barrier Coatings (EB-PVD TBC) and methods for applying the same to a substrate in general, and to such coatings and methods that utilize a thermally grown oxide for ceramic to metallic adhesion in particular.
2. Background Information
Thermal barrier coating (TBC) systems have been developed to fulfill the demands placed on current high-temperature Ni-base superalloys for gas turbine applications in both aero engine and land based gas turbines. TBC systems typically consist of a ceramic (e.g., yttria-stabilized zirconia) top layer that has low thermal conductivity, is chemically inert in combustion atmospheres, and that is reasonably compatible with Ni-base superalloys. The ceramic top layer is often applied by a deposition process such as Electron Beam Physical Vapor Deposition (EB-PVD). To ensure adequate bonding between the ceramic topcoat and the metallic substrate, it is common (but not required) to use a bond coat (e.g., NiCoCrAlY) disposed between the ceramic top coat and the metallic substrate. Ceramic adhesion to the bond coat depends on the formation of a thin, slow-growing oxide layer (also designated as TGO: thermally grown oxide) developing on the bond coat.
TGOs grown from a NiCoCrAlY or similar bond coat in a vacuum (at about 100 to 10−6 Torr) at temperatures less than 1800° F. will include certain oxides (e.g., eta phase alumina, and transition oxides, also referred to herein as “low temperature oxides”) that assume a voluminous, low integrity form that tend to have lower adhesion to the bond coat than other oxides. TBCs attached to these oxides will, therefore, be subject to these weaker bonds, and may be the basis for spallation.